It doesn't appear that the password in a text file was implemented in the standalone converters.
I tried both sqb2mtf.exe and SqbConverter.exe and was unsuccessful.
If you want to convert a file from the command line you could use the convert command from the command line.

The error indicates that: Password file does not exist: C:\\redgate\\dcr.txt
Is the file name and path correct?
It could also be permissions to the folder.
The logon account of the SQL Backup Agent have rights to the folder?

The syntax is fine.
I pretty much copied your command and it worked, I only changed the database name and the disk path.
The main thing it is compalining about is:
Password file does not exist: C:\\redgate\\dcr.txt

Try giving the log on account of the SQL Backup Agent explicit rights to the folder.